as mentioned, it all comes down to the operating temps under load. As long as you got a decent cooler to keep it under 60C, you're good.
I also have the Phenom II X3 720 BE (could they make the name longer plz), and right now it's running at 3.4GHz with no issues. That was originally done with an Artic Cooling Freezer 64 Pro http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Kept things nice and steady at 51C during Prime95
But now I'm liquid cooled, so I'm looking to push it up a little more.
Well after owning the X3 720 since last august I've done a lot of expirementing with it.
One thing that has been said before, you aren't going to get much past 3.7ghz. At least my chip was limited there. I was able stabilize pretty well at 202x18.5, 3737mhz @ 1.44V. Although don't get voltage happy, it may be able to handle all that voltage but it can make it more unstable.
Just keep the temps under control and you can push it to the limit. Unfortunatly your overclocking will be limited because of the chip. My liquid cooling setup kept it under 35C with the processor at 3.7ghz @1.4V, that was after 8hrs of prime95. This is just an example of how much you can cool it but it just won't go any further.
Generally, your not going to overclock an AMD chip too high as long as you keep temps down.
Sounds like all you've done is open the multiplier a bit. In that case you've got nothing to worry about, you should be fine.
What everyone is talking about is when you go deeper into the BIOS and make changes to voltages and bus speeds and such.
Not on my OC. I just bumped the multiplier (with most recent overclock). Originally, I just downloaded and used AMD's Overdrive tool, which did make some changes to the bus speeds (not the voltage, though).
If you're not sure about going the OC route, and your motherboard has an AMD chipset, just download Overdrive and let it do it's thing.